I. Field of the Invention
This invention relates to a heated fuser device as is commonly used in xerographic copying machines, and more particularly to a heated pressure fuser device including an improved heated fuser member.
II. Description of the Prior Art
In a typical xerographic process a photoconductor comprising a photoconductive composition coated on a rigid or flexible substrate is uniformly electrostatically charged in the dark, and then exposed by being illuminated in an image pattern in accordance with graphic material on an original document. The photoconductor becomes discharged in the areas exposed to the illumination, but retains its electrostatic charge in the dark areas, which areas correspond to the graphic material on the original document. The resulting electrostatic latent image is developed by depositing on the photoconductor a finely divided electrostatically attractable developing material (toner). The toner will normally be attracted to those areas on the photoconductor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This visible image of developing material is then transferred to a support surface, such as plain paper or any other suitable substrate, to become the ultimate copy. Any residual developing material remaining on the photoconductor is cleaned and the photoconductor is reused as described above for subsequent copies. The toner image that was transferred to the plain paper is then fixed thereto. Since the developing material is heat fusible, application of sufficient heat to the paper causes the developing material to melt and be fused into the paper so as to be permanently affixed thereto.
One basic approach to fusing in a xerographic copying machine is the use of the so-called hot roll pressure fusing apparatus in which toner particles are melted by the direct contact, under pressure, of a hot surface of a heated fuser roll with the toner particles. However, as the toner particles are heated, they soften and become sticky to the extent that they readily adhere to other surfaces. During a typical fusing operation there is a tendency for part of the heated toner image to stick to the heated fuser roll. The toner which adheres to the fuser roll will, of course, transfer to the next sheet of support material passing through the fuser, thus producing dirty copies. This process is commonly referred to in the printing art as "offset."
In an attempt to avoid the problem of offset during a fusing process, heated fusing rolls were developed either with (1) a thin outer layer of a tetrafluoroethylene resin (e.g. a tetrafluoroethylene resin sold under the trademark "Teflon" by E. I. DuPont de Nemours & Co.) and a silicone oil film, or (2) a thin outer layer of a silicone elastomeric material (silicone rubber) and a silicone oil film. The physical characteristics of the Teflon material or the silicone rubber with the silicone oil film are such that they are repellent to sticky or tacky substances.
Although the use of either of the above-described coated heated fuser rolls in a xerographic copying machine has helped to prevent the offset problem, they do present other disadvantages. For example, both of these types of coatings exhibit only moderate thermal conductivity properties. This creates various temperature gradients across a heated fuser roll, and thus uneven heating of the toner particles with subsequent poor fusing results. Furthermore, these types of coated fuser rolls present numerous problems in the methods for their manufacture. In addition, it is sometimes necessary to use a silicone oil film on these fuser rolls because there is some tendency of these rolls to pick up toner particles during fusing. To constantly apply a silicone oil film on the roll, the copying machine requires a separate oil container, an apparatus to apply the oil to the rolls as well as a metering and control system to maintain the proper amount of fluid on the roll. Thus there is the requirement of elaborate equipment and therefore additional expense to provide the silicone oil film. When silicone rubber is exposed to silicone oil, the silicone rubber can swell and the integrity of the rubber can deteriorate thereby decreasing its effectiveness under the pressures and temperatures normally encountered in pressure fusing systems. Silicone oil applied to the fuser roll can flow onto a silicone rubber coated pressure roll causing the same disadvantage unless the pressure rolls are end capped to prevent exposure of the silicone rubber layer to the oil. However, this precaution results in added expense in the cost of the pressure roll.